Early work relating to network-based Wireless Location Systems is described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in U.S. Pat. No. 5,608,410, Mar. 4, 1997, “System for Locating a Source of Bursty Transmissions.” Both of these patents are assigned to TruePosition, Inc., the assignee of the present invention. TruePosition has continued to develop significant enhancements to the original inventive concepts. Matched-replica processing is also described in U.S. Pat. No. 6,047,192, Apr. 4, 2000, “Robust, Efficient Localization System”.
Over the past few years, the cellular industry has increased the number of air interface protocols available for use by wireless telephones, increased the number of frequency bands in which wireless or mobile telephones may operate, and expanded the number of terms that refer or relate to mobile telephones to include “personal communications services,” “wireless,” and others. The air interface protocols now used in the wireless industry include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS, ESMR, GPRS, EDGE, UMTS WCDMA, WiMAX, LTE/SAE/eUTRAN and others.
As radio power levels decrease with increasingly strict power control schemes and with the introduction of advanced spread spectrum coding schemes (CDMA, W-CDMA, OFDM, SC-CDMA, etc) that require continuous efficient power control, the ability of a wireless location system to detect radio signals at neighboring and geographically proximate receivers is reduced. Location techniques used by the wireless location system can include: Time-difference-of-arrival (TDOA), Angle-of-Arrival (AoA), hybrid TDOA/AoA and hybrid terrestrial TDOA with Global Navigation Satellite System (GNSS) measurements. A current example of a GNSS system is the United States NavStar Global Positioning System (GPS).